The conversion of alcohols to higher molecular weight compounds—particularly industrially relevant alcohols—has been the subject of various investigations for many years. The self-condensation of an alcohol to an alcohol with double the molecular weight was first reported by Marcel Guerbet nearly 100 years ago. The formation of “Guerbet alcohols” requires very high concentrations of a strongly basic alkoxide metal salt. Separation of the alcohol product from the catalyst and potential recovery and recycle or disposal of the catalyst can be expensive and environmentally challenging. In addition, the coproduction of water can rapidly deactivate the catalyst if the water is not efficiently removed during the reaction. Solubility problems associated with the alkoxide metal salt can also lead to processing concerns.
Typically, higher-molecular-weight alcohols, such as 2-ethylhexanol (2-EH or 2-EH-ol), are produced by the aldol condensation of aldehydes, for example normal-butyraldehyde (n-butyraldehyde). The aldol condensation reaction utilizes a homogeneous sodium or potassium hydroxide catalyst. The thus produced unsaturated aldehyde is further reduced over a nickel catalyst in the presence of hydrogen to produce the saturated alcohol. Disadvantages of this process include coproduction of salts and the dependence on petroleum products needed for the generation of the aldehyde starting material through hydroformylation, the so-called OXO process.
Recent focus on the production of biologically based alcohols has thrust the production of commercially relevant chemicals from these “green” raw materials into the forefront. Of particular interest is the use of ethanol from fermentation of corn, sugar cane, and other plant materials. Butanol produced from fermentation by genetically engineered microorganisms, referred to as “bio-butanol”, is also of growing interest. Conversion of bio-based ethanol and butanol to butyraldehyde, 2-ethylhexanol, 2-ethylhexenal or 2-ethylhexanal are of particular interest.
A vapor phase process utilizing a heterogeneous catalyst would preclude the product/catalyst separation issues in classical Guerbet chemistry and the conventional 2-EH process. Other attempts at heterogeneous catalysts have focused on a single catalyst capable of carrying out Guerbet condensations. These catalysts have been plagued by either low conversion and/or low selectivity to the desired products. However, by utilizing a dual heterogeneous catalyst system in a vapor phase reactor, one can efficiently produce the Guerbet alcohol, the unsaturated aldehyde, or the saturated aldehyde with higher selectivity. Subsequent hydrogenation over a polishing bed can give the desired products—reduced alcohol or saturated aldehyde.